Treatment in BMP’s and LID’s for InfoSWMM and InfoSWMM SA

5.1 Treatment

5.1.1Background

Management of stormwater quality is usually performed through a combination of so-called “best management practices” (BMPs) and a form of hydrologic source control popularly known as “low impact development” (LID). Treatment of stormwater runoff, either by natural means or by engineered devices, can occur at both the source of the generated runoff or at locations within the conveyance network. Source treatment through LID is discussed in the next chapter. This section describes how SWMM models treatment applied to flows already captured and transported within a conveyance system.

Table 5-1, adapted from Huber et al. (2006), categorizes the different unit treatment processes used by various types of conveyance system BMPs. Ideally one would like to model these processes at a fundamental level, to be able to estimate pollutant removal based on physical design parameters, hydraulic variables, and intrinsic chemical properties and reaction rates. With a few exceptions, the state of our knowledge does not permit this, at least within the scope of a general purpose stormwater management model like SWMM. Instead one has to rely on empirical relationships developed from site-specific monitoring data.

Strecker et al. (2001) discuss the challenges of using monitoring data to develop consistent estimates of BMP effectiveness and pollutant removal. The International Stormwater BMP Database (www.bmpdatabase.org) provides a comprehensive compilation of BMP performance data from over 500 BMP studies on 17 different categories of BMPs and LID practices. It is continually updated with new data contributed by the stormwater management community. Table 5-2 lists the median influent and effluent event mean concentrations (EMCs) for a variety of BMP categories and pollutants that were compiled from this database. The cells highlighted in yellow indicate that a statistically significant removal of the pollutant was achieved by the BMP category. A summary of the median removal percentages of several common pollutants treated by filtration, ponds, and wetlands published in the Minnesota Stormwater Manual is listed in Table 5-3. Most of these percentages are consistent with those inferred from median EMC numbers in the BMP database table 5-2.

Table 5-1 Treatment processes used by various types of BMPs

Process

Definition

Example BMPs

Sedimentation

Gravitational settling of suspended particles from the water column.

Ponds, wetlands, vaults, and tanks.

Flotation

Separation of particulates with a specific gravity less than water (e.g., trash, oil and grease).

Oil-water separators, density separators, dissolved-air flotation.

Filtration

Removal of particulates by passing water through a porous medium like sand, gravel, soil, etc.

Sand filters, screens, and bar racks.

Infiltration

Allowing captured runoff to infiltrate into the ground reducing both runoff volume and loadings of particulates and dissolved nutrients and heavy metals.

I have noticed based on email questions and postings to the SWMM LIst Sever (a great resource hosted by CHI, Inc.) that many SWMM 5 users do not know about the really outstanding documentation on SWMM 5 posted on the EPA Website https://www.epa.gov/water-research/storm-water-management-model-swmm It consists of two now and in the near future three volumes on Hydrology, Water Quality, LID’s and SuDs and Hydraulics. The documentation is fantastically complete with detailed background on the theory, process parameters and completely worked out examples for all of the processes in SWMM5. It is truly an outstanding aid to modelers and modellers worldwide. It would benefit you to read them (if you have not already downloaded the PDF files). Thanks for reading this post